It was now certain that Bohemia was to be the theater of war. This province of the Austrian Empire may be described as a huge basin, whose rim is composed of mountains. It is separated from Silesia by the Riesengebirge (Giant Mountains), from Saxony by the Erzgebirge (Iron Mountains), from Moravia by the Moravian Hills, and from Bavaria by the Fichtelgebirge and the Böhmerwald; the Moravian Hills and the Böhmerwald separating it from the valley of the Danube. This great basin is drained by the Elbe river, which, rising in the Riesengebirge, makes a huge loop, flowing first south, then west, and finally north, and receives the waters of the Iser, Adler, Moldau and Eger rivers before it issues forth from the Bohemian frontier into Saxony. This theater is well suited to defensive operations, as the mountain frontiers are penetrated by few passes, and the forests and rivers constitute additional obstacles. On the Silesian frontier the only issues by which an invader can enter Bohemia are the passes of Trautenau, Eypel, Kosteletz, Nachod and Neustadt. These passes could all be easily defended, while on the Saxon frontier the passes of Reichenberg, Gabel and Königstein-Tetschen could be used by retarding forces, which could afterwards find a strong defensive line on the Iser. [Illustration: No. 2. 1st. ARMY ON 22ND., 23RD. & 24TH. JUNE.] Two railway lines lay in the theater of war, and were of great importance to the contending armies. One line ran from Vienna, _via_ Kosel, Breslau and Görlitz, to Dresden. The other connected the Austrian capital with Prague, _via_ Olmütz (or Brünn) and Bömisch Trübau. The two lines were joined by a railway from Dresden to Prague, and by one which, running from Löbau to Turnau, branched from the latter point to Prague and Pardubitz. These railways connected with others leading to all the important cities of Prussia. The two Prussian armies could cover their railway communications while advancing; but the Prague-Olmütz line, which was of vital importance to the Austrian army, ran parallel to, and dangerously near, the Silesian frontier, and was not covered by the Austrian front during the operations in Bohemia. The Prussian advance began on the 20th of June. The Army of the Elbe marched from the vicinity of Dresden, _via_ Stolpen, Neustadt, Schluckenau and Rumburg, to Gabel. As the greater part of this march had to be made by one road, it required six days, though the distance was only 65 miles. The First Army had concentrated at Zittau, Herrnhut, Hirschfelde, Seidenberg and Marklissa. From these points it began its march on the 22d of June, each division marching by a separate road; and on the 25th it was closely concentrated around Reichenberg. The entire Prussian front was now reduced to about 100 miles, and Herwarth Von Bittenfeld was only twelve miles from Frederick Charles.